Semi-conductor alternating current regulating means

ABSTRACT

The specification discloses an alternating current regulating means, for use for example in a temperature control system, by means of which an alternating current flowing in a load circuit is controlled in an ON/OFF manner in response to and in accordance with a small DC control signal such as may be provided, for example, by a process computer. The regulating means includes a semi-conductor switching means controlled by a control means which is responsive to a converted DC control signal obtained from a signal convertor which provides electrical isolation between the DC control signal source and the switching means. The control means is also controlled so that the switching means is rendered conductive only when the AC supply voltage is of substantially zero value, and so that only an even number of half-cycles of current is passed. The DC control signal may be of analogue or digital form, and may be provided in the latter case by electrical relay contacts.

United States Patent 1191 Detmann et a1.

1111 3,749,884 1451 July 31 1973 SEMI-CONDUCTOR ALTERNATING CURRENT REGULATING -MEANS [75] Inventors: Walter Detmann, Niedervalluf;

Gerhard Meid, Pfungstadt; Klaus Simon, Okriftel, all of Germany [73] Assignee: Kalle Aktiengesellschait,

Wiesbaden-Biebrich, Germany [22] Filed: Dec. 17, 1971 21 Appl. No.: 209,194

[52] US. Cl. 219/501, 219/494 [51] Int. Cl. "05b 1/02 [58] Field of Search 219/494, 497, S01; 323/3 C, 18, 22 SC, 24

[ 56] References cited' UNlTED STATES PATENTS 1 3,300,622 1/1967 Swain..- 219/497 3,371,191 2/1968 Seney...'..' 219/497 X 3,519,917 1 7/1970 Martin 219/497 X 3,586,830 6/1971 Leitner 219/497 X 3,646,577 2/1972 Ernst 219/501 X 3,651,753 3/1972 Schmidt 2'19/501 x Primary ExaminerBemard A. Gilheany Assistant Examiner-F. E. Bell Attorney-James E. Bryan [57] ABSTRACT The specification discloses an alternating current regulating means, for use for example in a temperature control system, by means of which an alternating current flowing in a load circuit is controlled in an ON/OFF manner in response to and in accordance with a small DC control signal such as may be provided, for example, by a process computer.

The regulating means includes a semi-conducto I 6 Claims, 6 Drawing Figures PHASE r28 CONDUCTOR T I 0.0mm: 3

CONVERTOR 1 I25 I g [24 I27 77 SWITCH CONTROL 1 TEMPEWURE AMPLlF/ER/ L040 1 CONTROLLER 'gg gg Mam SWITCH (PULSE SWITCH) 1 691 592.51 1 wee 251.... 27' MEANS g YNCHRON/S/NG 1 STAGE 2? NEUTRAL 25 CONDUCIUR SEMI-CONDUCTOR ALTERNATING CURRENT REGULATING MEANS electrically-heated industrial control systems'there is a problem of exactly measuring out the energy input-by a suitable regulating means in dependence upon the output signal of the control system error detector.

This can be performed by the use of an electromagnetic relay with which the heating tapes or coils, connected directly to the AC supply voltage, are switched on and off, the relay being controlled by a two-state controller.

Since the introduction of uniand bi-directional thyristorsit-is possible-to avoid the disadvantages of such relays, as for example, the noise, high energy consumption, and above all the limited operational life, and to switch alternating currents by means of semi-conductor regulating means having high reliability. Some circuit arrangements operate in an on/off manner, withswitch ing occurring at random phase-angle.

Other arrangements which permit continuous variation of the load control the phase angle of switching.

Switching on at both random and controlled phase angles generates high transient voltages which can jeopar dize the reliability of complex electronic installations, quite apart from'the fact thatun'der ce'rtain circumstances the thyristors themselves are'su bjected to impermissably high stresses. 1

lnwhat follows the individual methods-and circuit arrangements, andtheir particular defects will be considered in more detail.

The advantages of switching-on at a controlled phase angle are obvious, since this requires only a simple circuit arrangement for controlling the power semiconductor device at the highest possible regulating frequency. There are, however, considerable disadvantages. The crystal of the Triac or Thyristor is subjected to increased stresses which must be taken into consideration when sizing these devices. Furthermore, considerable transient noise voltages are generated, for the suppression of which additional expenditure is necessary. Particularly for those who employ process computers it is of great importance to maintain transient noise voltages as small as possible, in order to avoid interference. Moreover, even with a resistive load a reactive current dependent on'v the control phase angle is produced. In a larger installation this is not a factor to be ignored when planning the electrical equipment.

On account of these disadvantages phase angle control for the regulating means, particularly for electrical heating arrangements, cannot be considered.

Pulse gating operates by the formation of a running average value. In this control method the average value of the switched power is continuously formed, and this is compared with the input signal-by a feedback circuit.

The error signal so produced causes the circuit each time on demand to switch on for a whole cycle'of the supply voltage, until equality between the input signal 1 a regulating ratio of 50 percent, switchingon occurs; ,every 40 milliseconds. Thislmeans that the high tran and the average value of the output power is reached.

The method of pulse gating is, after phase angle con trol, the best approximation to continuous control. It is well suited to resistive loads, but in the case of inductive load devices the power semi-conductor device is more highly stressed than with pulsegroup switching. For inductive loads an increased transient current arises on switching on. In the case of pulse group switching, with a cycle time of 2 seconds, switching on occurs only every 2 seconds, whilst with pulse gating through the formation of the running average value, for

' sie'nt current of the power semiconductor device mu be overcome almost continuously.

Controlled switching of the power semiconductor device is important for the electrical stressing of the de vice.

- avoided, and with resistive loads no reactive load is generated.

Against this, greater expenditure on'the control circuit is incurred, and a smaller control frequency is ob Y tainable. r

"Switching on at the zero voltage point in the case of pulse group switchingis preferable, but in the case of pulse gating nothing else can beconsidered.

lt'is therefore the object of the =present'invention to; provide a c'ontactless regulating means which is universally applicable, and which for the'avoidanc'e of the disadvantages of the abovementioned circuit arrangements will fulfil the'followingrequirements.

'One requirement is that the AC supply network shall not vbe loaded with'direct current. Triacs'or Thyristors' change from the conductingto the'non-conducting state when the current falls to zero value. Random switching of positive or negative h'alf-wavesthasas a possible consequence that an unequal number of positive and negative current half-waves may be taken from the supply source. In such a case a direct current coin ponent may arise as average value, and this can lead to polarised magnetisation of transformers in the power supply system. If this is to be avoided, only complete sine waves should be switched. 1

Spurious switching-on of the Triac or Thyristor by voltage spikes must be reliably prevented. These spikes can come from the supply network, or be caused by the power semi-conductordevice itself on switching off an inductive load.

By suitable means the rate of voltage rise across the power semi-conductor device may be limited to the permissible value.

The Triac or Thyristor is operated as a rule near to its. permissible load limit, so that protection against short-circuitand overload is necessary. a I A further feature of .recenttimes is the tendencyto control as many as possible industrial processes by pros cess' computer. Thus, for example, the use of process computers for the automatic control of plastics manufacturing machines is ever increasing. To keep'costs' low alternating current regulating means should be". controlled directly by the computer. For this purpose the electro-magnetic relay is unsuited, since it needs a large operating power. Hence power amplifying means Switching on of the power semi-conductor at the zero must be used between the output of the computer and the relay.

It thus appears desirable to provide an alternating current switch on an electronic basis which can be directly energised by the computer, and whose life is independent of the number of switching operations performed.

The use of process control systems in larger installations is increasing more and more. For the sake of uniformity the temperature controlling circuits of the extruder, for example, should be equipped with similar automatic control systems. The output signal of such control systems is a continuous modulated'direct current of between and milli-amps. However, no digitally working regulating means can be controlled by such an output signal.

Another difficulty is that resistive as well as inductive heating arrangements are to be found. The majority of electronic regulating means available on the market can handle only purely resistive loads. Thus apparatus is needed which can also handle inductive loads with a power feature down to about 0.7.

Accordingly, the present invention provides a semiconductor alternating current regulating means (which is particularly suitable for the control of heating arrangements) in which the load is connected through a load switch by means of a semi-conductor regulating means, in integrated form, comprising an input amplifier/comparison stage, a pulse switch, and a synchronising'stage which feeds back the voltage drop across the load to the pulse switch, and which is distinguished in that a. the input amplifier stage of the integrated circuit has connected thereto on its input side for digital input control aDC voltage convertor via an electronic control device,

for proportional input control a DC voltage convertor and a saw tooth generator, and for contact input control a suitable matching circuit; 1

b. switching occurs only at the zero voltage points of the AC supply voltage and only after even numbers of supply half-waves; and

c. electrical separation between the load and signal circuits is obtained by a single transformer.

In a preferred arrangement, particularlyfor the control of inductive loads, switching of the supply voltage half-waves results from pulse group switching.

The parts employed will now be described in more detaiLThe standard electric signal of the control art is converted in a DC voltage convertor into a DC voltage corresponding to the circuit. The DC voltage convertor itself.consists of a blocking oscillator with a transformer. The resulting AC voltage is taken from the secondary winding of the transformer, and is rectified, and smoothed. The test voltage between primary and secondary windings can be varied; it amounts according to the relevant safety regulations of the present time to 2.5 kilo-volts.

A semi-conductor module with pronounced threshhold and negative characteristics, in combination with a time determining circuit (resistors and condensers) is used as a saw-tooth generator.

The charging action of the module is made linear through a suitable arrangement which delivers a constant charging current. The charging of the condenser is thus strictly proportional to time. When the voltage of I and a cycle time of 2 seconds a resolution dependent the condenser reaches the thresh-hold voltage, predetermined in the semi-conductor module, the condenser suddenly discharges, the semi-conductor module blocks, and the charging process begins anew.

For the cycle time of this charge/discharge process one chooses a time which is greater than the thermal time constant of the control circuit. The cycle time can be adapted by external circuitry of the semi-conductor element to different requirements. As according to the invention only an even number of successive supply half-waves should be switched, one can achieve with a supply frequency of, for example, cycles per second upon the input voltage of l percent.

A regulating means according to the present invention is suitable for switching slightly capacitive, purely ohmic, and inductive loads at any powerfactor.

The ability to switch inductive loads is particularly required, for example, in the field of eddy current heat- Although all control operations can be carried out with the regulating means according to the present invention, as for example in motor and magnetic valve control, this regulating means has in practice proved itself particularly in the control of temperature of melts. For thermo-plastic melts the temperature control of extruders, pipes and nozzles in particular has been controlled by control present invention. 1 7

One alternating current regulating means according to the present invention as applied to an automatic heating system for an extruder of plastics materials will now be described in greater detail by way of example and with reference to the drawings in which! FIG. 1 shows schematically the general arrangement of the heating system and extruder;

FIG. 2 shows a block diagram of the electrical apparatus of the heating system; for control by means of an analogue control signal; I

FIG. 3 shows a block diagram of the electrical apparatus of the heating system, for control by means of a digital control signal;

FIG. 4 shows a block diagram of the electrical apparatus of the heating system, for control by means of an electrical relay contact;

FIG. 5 shows a circuit diagram of a control signal converter forming part of the electrical apparatus shown in the FIG. 2; and I I FIG. 6 shows a circuit dia'gramof an auxiliary control means forming part of the electrical apparatus shown in the FIG. 2.

Referring now to the FIG. 1 of the drawings an extruder 10 includes a nozzle 11 and a supply pipe 12 for conveying plastics material in molten form to the nozzle for extrusion. In order to maintain the plastics material at the correct temperature for extrusion the supply pipe is provided with an automatic electrical heating system which includes a plurality of heating coils 13 wrapped closely around the supply pipe and'connected for supply with alternating current from the rspective phases L,S,T, of a three phase supply source 14. The supply of alternating current to the respective heating coils is controlled by a plurality of alternating current regulating means 15 acting under the common control of a temperature controller 16. This controller includes adjustable means (not shown) for providing a temperature reference signal, and receives a temperature demeans according to the pendent feedback signal from a temperature detector 17 housed in an insulating jacket 18 which encloses the supply pipe. The controller supplies at its output signal circuit 19 a temperature error signal which is dependent on the deviation of the actual temperature from the desired value set by the referencesignal, and this error signal .is supplied to the respectiveualternating' current regulating means as a control signal.

The temperature feedback signal, the-temperature reference signal, and the temperature error'signal are all analogue signals.

;Each regulating. means ists hovvn more detail .FIG. 2 in which the heating coils of the particular phase are shown as a load 20 connected in series with a semiconductor load switch 21 between the neutral conduc- I tor 22 and the appropriate phase conductor 23 of the supply source. The load switch 21 comprises a bidirectional silicon controlled rectifier (SCR), and will be referred to hereafter in this description as a triac.

Control pulses for renderingthe triac conductive are derived from a switchcontrol means 24 in response to input signals provided by an amplifier 25,. and under the control of an auxiliary control'means 26.

The amplifier receives input signals from two sources, namely from (a) a saw-tooth generator 27 and (b) a control signal convertor 28.v I v This convertor in turn receives as its input signals control signals from the temperaturecontroller 16, and

the controller receives as itsinput signals temperature dependent feedback signals from thetemperature detector l7, and its-internaltemperature reference signal. v

The auxiliary control means receives a signal dependent on the AC voltage. appearing across the triac 21,

and operates in response thereto toprovide synchronis-- ing signals so that the switch, control means 2 4,.in re- In this FIG. 6 the switch control means 24 includes a series circuit 35 comprising an SCR 36, a resistor 37 and a capacitor 38, the SCR being connected adjacent the neutral conductor 22, and being controlled by the amplifier 25. A circuit connecting a point between the resistor 37'and capacitor 38 and the trigger electrode 39 of the triac 21- includes a parallel circuit which has in one branch circuit a Zener diode 40 and an SCR41, and in the otherbranch circuit an SCR 42.

A Zener diode 43, operating in conjunction with a resistor 44, serves to limit the voltage to be developed I across the'series circuit 35 which includes the capacitcr; 1

tor 38 receives charge during each-.negativehalf cycle, i.e., when the neutral conductor 22 is positive with re-,

spect to the phase conductor 23. At the beginning of each negative half cycle the transistor 49 becomes con. ductive'to trigger the SCR 42 whereas at the'beginning of each positive half cycle the transistor .50 becomes conductive to trigger the SCR 41. j

The capacitor 38 is charged sufficiently duringeach negative half cycle to enable theSCR41 to be triggered during the succeeding positive half cycle, so that even after the output signal of the amplifier 25 has-ceased to hold theSCR 36. conductive, conduction of the triad 21 sponse to a signalfrom the amplifier 25, supplies con- I trol pulses to thetriac only when the' AC voltage across the triac is of substantially zero value, and so that on termination of' the control signal from the amplifier 25 conduction of the triac is maintained until thenext end plifier 25, and of providing electrical isolation of the incoming control signal circuit and the outgoing converted control signal circuit. In this way the delicate low voltage circuitryofthe temperature controller 16 is electrically isolated; from the high voltage circuit of the load circuit 20 andits' switching and associated control means. j l a v The convertor comprisesa blocking oscillator circuit 29, with transformer coupling 30 between the base and is ensured in the positive half cycle. Thus theitriac is heldc'onducting always foran even number of half cycles.

Though in FIG. 1 the control signal convertor 28jhas been supplied with an analogue signal, it could be" fed directly with digital control signals, in which case the saw-tooth generator 27 is no longer necessary and is omitted. Such an arrangement is illustrated in the F IG. 3,where the digital controlsignlas are derived from an output transistor circuit 53 of a digital temperature controller. Such a digital controller may comprise a digital computer which processes the feedback and reference signals to provide a digital error signal for controlling the alternating current regulating means in a temperature corrective sense.

manner by an output electrical relay contact of the temperature controller, the contact being opened and closed au'tomatically'so that the time average of the closed periods is proportional-to thetemperature error collector circuits of a transistor 31. A third-winding 32 V on the transformer 30' provides the output signal (conj The auxiliary controlmeans 26 hasa circuit as shown in FIG. 6, which also showstypical constituent parts of the load switch 21, and theswitch control means 24.

signal. This arrangement isldisclose'din the FIG. 4. In this case the control signal convertor may need tofinelude suitable matching circuit elementsxThe relay contact is shown at 54. y

We claim: c 1. Alternating current regulating means for use in an electric control system, comprising:

semi-conductor switch means for connection with an alternating current supply source and a load circuit which is to be controlled by the control system;

switch control means connected to the semiconductor switch means for supplying thereto control pulses for enabling the switch means to become conductive and allow alternating current to flow in the load circuit;

control signal conversion means connected to the.

switch control means for converting direct current input control signals supplied by the control system into converted signals which are dependent thereon but are electrically isolated therefrom, the converted signals causing the switch control means to supply control pulses to the switch means,

first auxiliary control means connected to'the switch control means for ensuring that in the presence of a converted control signal the switch control means supplies control pulses to the switch means only when the instantaneous value of the alternating current supply voltage is of substantially zero.

value; and second auxiliary control means connected to the switch control means for ensuring that the conduc-' tive state of the switch means, induced by the presence of a control signal, is maintained until the first end of an even half cycle of current flow in the load circuit following the termination of that control signal, whereby conduction ofalternating current in i the load circuit in response to the presence of a control signal occurs for an even number of current half-cycles only.

2. Alternating current regulating means according to claim 1 wherein the control signal conversion means includes a blocking oscillator having an input circuit for receiving the direct current control signals and an nected to the control signal conversion means for sup-. 'plying thereto the direct current control signals; and

including connected between the control signal conversion means and the switch control means a signal combining means for combining the converted control signals with a saw-tooth reference signal from a saw-tooth generator, whereby to supply the switch control means with digital signals.

5. Alternating current regulating means according to claim 2 including an electrical switch contact and a sig-' nal matching'circuit connected to the'control signal conversion means whereby to supply the direct current control signals.

6. A temperature dependent control system for an electrical heating system having heating elements connected for supply from an alternating current supply source through an alternating current regulating means according to claim 1, the said control signal being a signal dependentv on the temperature achieved by the heating system. 

1. Alternating current regulating means for use in an electric control system, comprising: semi-conductor switch means for connection with an alternating current supply source and a load circuit which is to be controlled by the control system; switch control means connected to the semi-conductor switch means for supplying thereto control pulses for enabling the switch means to become conductive and allow alternating current to flow in the load circuit; control signal conversion means connected to the switch control means for converting direct current input control signals supplied by the control system into converted signals which are dependent thereon but are electrically isolated therefrom, the converted signals causing the switch control means to supply control pulses to the switch means, first auxiliary control means connected to the switch control means for ensuring that in the presence of a converted control signal the switch control means supplies control pulses to the switch means only when the instantaneous value of the alternating current supply voltage is of substantially zero value; and second auxiliary control means connected to the switch control means for ensuring that the conductive state of the switch means, induced by the presence of a control signal, is maintained until the first end of an even half cycle of current flow in the load circuit following the termination of that control signal, whereby conduction of alternating current in the load circuit in response to the presence of a control signal occurs for an even number of current half-cycles only.
 2. Alternating current regulating means according to claim 1 wherein the control signal conversion means includes a blocking oscillator having an input circuit for receiving the direct current control signals and an output circuit which is transformer-coupled with the input circuit, and rectifying means connected to the oscillator output circuit for rectifiying the magnetically-induced output circuit signals, the latter signals being supplied as input signals to the switch control means.
 3. Alternating current regulating means according to claim 2 including a digital control circuit connected to the control signal conversion means for supplying thereto the direct current control signals.
 4. Alternating current regulating means according to claim 2 inclUding an analogue control circuit connected to the control signal conversion means for supplying thereto the direct current control signals, and including connected between the control signal conversion means and the switch control means a signal combining means for combining the converted control signals with a saw-tooth reference signal from a saw-tooth generator, whereby to supply the switch control means with digital signals.
 5. Alternating current regulating means according to claim 2 including an electrical switch contact and a signal matching circuit connected to the control signal conversion means whereby to supply the direct current control signals.
 6. A temperature dependent control system for an electrical heating system having heating elements connected for supply from an alternating current supply source through an alternating current regulating means according to claim 1, the said control signal being a signal dependent on the temperature achieved by the heating system. 